1. Field of the Invention
The present invention relates generally to power supply circuits. More particularly, the present invention relates to a power supply circuit comprising at least one transformer which is connected to a primary side circuit and to a secondary side circuit. Furthermore, the present invention relates to a method for producing such a power supply circuit.
2. Description of the Related Art
Power supply circuits, such as DC power supplies, are nowadays designed as linear 50 Hz transformers or as switched-mode mains power supplies with switching frequencies within a range of a few kilohertz up into the megahertz range. The general attempt has been made to reduce the volume of the circuit, and thus of the device, to increase user friendliness especially in chargers for mobile applications. With the configuration as a switched-mode mains power supply and the resulting omission of the relatively large and heavy 50 Hz transformer, a first step has been made towards a reduction of the size.
With the present designs of power supply circuits in which use is made of a two-dimensional printed circuit board according to FIG. 9 and of corresponding connector housings and output lines, miniaturization is however very limited. For instance, integration into the device to be power-supplied, i.e. the load, is most of the time not possible. The power density of today's low-power supply units is about 2 to 3 W/inch3.
Furthermore, miniaturization is ruled out by the use of conventional discrete components, such as electrolyte capacitors or discrete resistors. Such components have specific minimum dimensions that must be adhered to even in cases where the typical values of said component are reduced. An alternative which allows smaller dimensions would be desirable.
International patent application WO 99/43074 discloses an arrangement in which a circuit for converting mains voltage into a lower voltage is accommodated in an output connector of the power supply device. Power supply circuits that have so far been realized have, however, a volume that is too large for realizing said arrangement in a really user-friendly way. Therefore, a corresponding miniaturization of the power supply circuit would be needed for use in such a device.
In the known constructional form of the plug-in power supply unit with output line, there is a power loss that cannot be avoided on the output line because relatively high power is present at the output side and the output line cannot be designed with any desired large cross-section for reducing the ohmic resistance accordingly. As shown in DE 100 18 229 A1, the output voltage is e.g. controlled at the primary side, so that only a few components are needed at the secondary side. A load-dependent voltage drop on the secondary line is bound to occur in this arrangement. This leads to unavoidable inaccuracy in the output voltage on the output connector because the voltage drop on the line cannot be measured during operation and is thus not available for controlling the output voltage. An omission of the output line, which is possible with a corresponding miniaturization and accommodation of the circuit in the output connector of the device according to WO 99/43074, would eliminate said drawback.
Furthermore, to ensure a standardized electrical isolation of the primary circuit and the secondary circuit, air and leakage paths must be observed on the circuit carrier. In the case of a two-dimensional printed circuit board, as shown in FIG. 9, this always entails a great distance between the components of the primary side and those of the secondary side. Therefore, components must not be placed on the area marked by reference numeral 101 in FIG. 9. The distance must e.g. be at least 6.4 mm upon application of the standard EN60950 when the mains voltage is 230 V. Therefore, there is a need for more flexibility in the configuration of the air and leakage paths.